The present invention relates to a printing head and, more particularly, to a printing head used in a wire dot-matrix printer.
A printing head of this type generally comprises a housing section and a core section, which are fastened together by screws. The housing section includes a housing and an armature guide fixed thereto. A number of armatures are supported by the armature guide, and a stopper is mounted on the guide, so as to face the armatures. Printing wires protrude individually from the armatures, so that their distal ends are arranged in two lines. The housing is provided with springs which urge the armatures toward the stopper.
The core section includes a plurality of cores, equal in number to the armatures, a plurality of coils wound individually around the cores, and a circuit board for controlling the current supplied to the coils. These elements of the core section are housed in a case. When the core section is coupled to the housing section, the armatures face their corresponding cores, and are partially pressed against the cores by a press ring in the housing section. Thus, each armature is rockable around the point of contact with its corresponding core. As the armatures rock in this manner, their corresponding printing wires reciprocate axially.
When, in printing operation, selected coils are energized by means of the circuit board, a magnetic flux is produced in their corresponding cores. The armatures corresponding to these cores are attracted thereto by the magnetic flux. Thus, the armatures rock toward the cores, against the urging force of their corresponding springs, so that the printing wires project from the housing, thereby effecting the desired printing on recording paper. When the coils are deenergized, the armatures are urged by the springs to rock to their initial position where they abut against the stopper. Thereupon, the printing wires are disengaged from the recording paper.
Being constructed in this manner, however, the aforementioned conventional printing head has the following drawbacks:
In the course of returning to the initial position, urged by the springs, the armatures normally strike against the stopper, thereby subjecting the stopper to a considerable impact. Since the stopper is attached to the armature guide, such an impact acts also on the guide. Therefore, the guide must be sufficiently sturdy to withstand the impact. Thus, the armature guide and hence, the printing head, cannot easily be made compact. Since the stopper has its end portions supported by the armature guide, moreover, it is liable to vibrate heavily, thereby producing noise, when subjected to the aforesaid impact. The rocking angle of the armatures, which determines the stroke of the printing wires, depends on the relative positions of the stopper and those portions of the cores in contact with the armatures. In the case of the printing head constructed in this manner, the cores and the stopper are provided at different parts of the head, that is, the core section and the housing section, respectively. It is therefore difficult to achieve the desired positional relationship between the cores and the stopper with a sufficiently high degree accuracy, or to accurately determined the stroke of the printing wires. While the printing head is operating, furthermore, the cores produce heat, which is transmitted to the stopper through the armature guide and the armatures. Having the aforementioned construction, however, the stopper has a low effeciency of heat radiation; consequently, it may possibly be deformed by the heat. If the stopper is deformed in this manner, then the armatures cannot be positioned satisfactorily, and accurate printing operation cannot be accomplished with ease.